Phosphors are used in such apparatuses as fluorescent lights, vacuum fluorescent displays (VFD), field emission displays (FED), plasma display panels (PDP), cathode-ray tubes (CRT), and white light-emitting diodes (LED). In any of these applications, a phosphor requires the supply of excitation energy for emitting light. After being excited by a high-energy source that emits vacuum ultraviolet light, ultraviolet light, visible light, electron beams or the like, such a phosphor emits ultraviolet light, visible light, or infrared light. However, there has been a problem that the long-term exposure of a phosphor to such an energy source may result in the deterioration of brightness.
In response to this, many novel ternary or more complex nitrides have recently been developed as alternatives to known phosphors, such as silicate, phosphate, aluminate, borate, sulfide, and oxysulfide phosphors. In particular, recently developed silicon nitride-based multicomponent nitrides and oxynitrides exhibit excellent characteristics as phosphors.
Patent Document 1 discloses phosphors represented by the general formula MxSiyNz:Eu (M is one or more alkaline earth metal elements selected from the group consisting of Ca, Sr, and Ba, whereas x, y, and z are numbers that satisfy the relationship expressed as z=2/3x+4/3y). Such phosphors are synthesized by nitridation of alkaline earth metal elements and then mixing the obtained alkaline earth metal element nitrides with silicon nitride or by heating alkali earth metal imides and silicon imides as raw materials under nitrogen or argon flow. Both synthetic methods require alkaline earth metal element nitrides susceptible to air and moisture as raw materials, and thus are unsuitable for industrial manufacturing.
Patent Document 2 discloses oxynitride phosphors having an oxynitride represented by the formula M16Si15O6N32:Eu and oxynitride phosphors having SiAlON structures each represented by the formula MSiAl2O3N2:Eu, M13Si18Al12O18N36:Eu, MSi5Al2ON9:Eu, or M3Si5AlON10:Eu. It states that, particularly in the case where M was Sr, heating the mixture of SrCO3, AlN, and Si3N4 at a ratio of 1:2:1 in a reducing atmosphere (hydrogen-containing nitrogen atmosphere) resulted in the formation of SrSiAl2O3N2:Eu2+.
This approach provides oxynitride phosphors only and thus does not provide phosphors based on nitrides free from oxygen.
Furthermore, raw materials of the nitride or oxynitride phosphors described above have low reactivity in a powder form. Thus, to promote the solid state reaction between the particles of the raw materials during firing, the raw materials should be heated with the maximum contact area between particles thereof. As a result, the synthesized phosphor is in the state of being compacted at high temperatures, in other words, in the state of a very hard sintered body. Such a sintered body should be pulverized into fine particles, which is a form suitable for its intended purposes as a phosphor. However, milling such a hard sintered body of a phosphor for a long period of time with tremendous energy in an ordinary mechanical method, for example, with the use of a jaw crusher or a ball mill, would result in the generation of many defects in the matrix crystal of the phosphor and thereby lead to the significant deterioration of the light emission intensity.
Meanwhile, the patent documents state that, in the production of such nitride or oxynitride phosphors, alkaline earth metal element nitrides such as calcium nitride (Ca3N2) and strontium nitride (Sr3N2) are preferably used. However, in general, divalent metal nitrides are likely to react with water to produce hydroxides and thus unstable under a water-containing atmosphere. This tendency is marked especially in the particles of Sr3N2 and metallic Sr, so these kinds of nitrides are very difficult to handle.
For the reasons described above, novel raw materials of phosphors and methods for producing them are demanded.
A method for producing a nitride phosphor using a metal as a starting material has recently been reported in Patent Document 3. Patent Document 3 discloses an example of a method for producing an aluminum nitride-based phosphor and describes that a transition elements, a rare earth element, aluminum, and an alloy thereof can be used as the starting materials. However, this patent document describes no example in which such an alloy is actually used as a starting material but describes that metallic Al is used as an Al source. This method uses a combustion synthesis technique in which a starting material is rapidly heated to a high temperature (3,000 K) by igniting the starting material and therefore is significantly different from a method according to the present invention. It is probably difficult to produce a high-performance phosphor by this method. More specifically, the method in which the starting materials are instantly heated to a temperature as high as 3,000 K has difficulties in distributing activator elements evenly and thus cannot easily provide a high-performance phosphor. This document describes no nitride phosphor containing an alkaline-earth element obtained from the alloy or no nitride phosphor containing silicon.    Patent Document 1: PCT Japanese Translation Patent Publication No. 2003-515665    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-206481    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2005-54182
The inventors have conducted studies and found that in the case of the production of a phosphor composed of a nitride matrix or an oxynitride matrix from an alloy as a raw material, a nitridation reaction proceeds rapidly during heating, so that the generated heat may cause melting and phase separation of the raw material and decomposition of the resulting nitride, thereby degrading the properties of a phosphor produced. The inventors also found that in particular, in the case where a large amount of the raw material is heat-treated at one time or the packing density of the raw material is increased in order to increase the productivity, a phosphor is not produced in some cases.